Helical scanning has been widely used to magnetically record analog and digital information on magnetic recording tape. In a helical scanning apparatus, the magnetic recording tape is helically wrapped around a helical scanning drum. Typically, a helical scanning drum comprises a drum body and a drum cover. The drum body includes a rotatable drum portion and may also include a stationary drum portion. One or more transducing heads are mounted on the periphery of the rotatable drum portion in a circumferentially spaced relationship. Each transducing head protrudes through a head port and actually pushes into the surface of the magnetic recording tape. The poles of the transducer heads are separated by a microscopic air gap. It is across this gap that the magnetic flux lines, which are representative of the information to be recorded or played back, are either generated or sensed.
During operation, the rotatable drum portion rotates about its longitudinal axis at high speed, e.g., at a speed in the range from 1800 to 3600 rpm. As the rotatable drum portion rotates, a pressurized air film is generated beneath the tape. The air film minimizes friction between the magnetic recording tape and the helical scanning drum. In some helical scanning drums, the drum cover is provided with an air vent disposed in a centered position on the drum cover. One example of an apparatus having a helical scanning drum with a vented drum cover is commercially available as VPR-80 Type "C" helical video recorder from Ampex Corp. Helical scanning drums have been generally described, for example, in U.S. Pat. Nos. 3,404,241; 3,691,315; 4,184,183; 4,414,588; 4,909,110; 4,916,555; 5,003,413; and 5,003,424.
Any contamination of the transducer head or the air film with debris can cause spacing loss between the tape surface and the head gap, preventing the recording or playback of a certain amount of magnetically stored information for a certain period of time. Such a signal loss is referred to as a "dropout". Dropouts have been particularly troublesome in helical scanning drums of the type in which the drum cover is provided with an air vent.